The present invention relates to mold release agents. More particularly, it relates to a semi-permanent water-based mold release agent for composite materials.
Molding is a well known technique for producing finished parts made from composite materials such as graphite-containing materials or fiber reinforced polyester materials and the like. Molded graphite-containing composite parts are particularly prevalent in the aerospace industry. For example, many airplane hulls (i.e. fuselage and wings) are constructed from a multitude of molded composite parts that are subsequently bonded together using an adhesive.
Typically, layers of fiber-mat material that have been impregnated with a liquid epoxy resin composite (such as prepreg plies) are laid down to conform to the shape of a mold. The mold is then heated to elevated temperature (e.g. 100-200° C.) to cure the liquid resin composite and provide the finished part. After the mold has cooled, the finished molded composite part is removed and the mold may be used again. In order to prevent the finished molded part from sticking or adhering to the mold surface, a mold release agent is applied to the mold surface prior to applying the prepreg plies to the mold. This process is well known in the art. It is important for molded composite parts that they can be demolded gently, using minimal force because, especially for the aerospace industry, the parts often are quite large and delicate. This means the mold release agent must provide superior release characteristics.
In certain applications, particularly when molding very large parts such as airplane wings, the molds are prepped at room or ambient temperature, and then inserted into very large ovens to cure the liquid resin to form the finished molded part. Consequently, the molds themselves, which are quite large, are not equipped with any heating equipment. This means that the mold release agent applied to the mold surface at room temperature must also be curable at room temperature in a relatively short period of time, such as a few hours. Otherwise, if one had to place the mold in the large ovens to cure the mold release agent, much time, effort and productivity would be lost. Once the mold release agent has cured at room temperature, it must still withstand the high temperatures (e.g. up to 300-400° C.) necessary to subsequently cure and form the molded parts.
Fiber reinforced polyester (FRP) parts are generally molded at around ambient temperature (10-70° C.). Most of FRP parts require a high gloss finish. This has been achieved by using the conventional and well known Frekote™ brand (available from Henkel Loctite) solvent-based semi-permanent mold release agents. However, certain FRP releases require their parts to have a matte finish or to have no transfer of mold release agent to the molded parts, such as “sanding gelcoat”. Conventional water based mold release agent such silicones and waxes are useful to release PRP parts but result in detrimental transfer of the mold release agent to the molded parts. Commonly used solvent based semi-permanent mold release agents are popular in the market place but do not provide a matte finish on the released parts. Solvent based release agents also contain hazardous VOCS.
There are two basic types of mold release agent, sacrificial and semi-permanent. A sacrificial mold release agent is one that is consumed or otherwise completely depleted after a single molded part is made in the mold. Known sacrificial mold release agents include natural and synthetic compounds such as silicone oils, mineral oils, waxes, fatty acid derivatives, glycols, etc. Sacrificial mold release agents must be constantly reapplied to the mold surface prior to making each molded part. For this reason, sacrificial mold release agents are costly and cumbersome to use. Also, molded composite parts are usually post bonded (adhered to other composite parts using an adhesive). Transfers of sacrificial release material will affect the bonding ability of the adhesive. Hence, sacrificial mold release agents are not suitable for molding many composite parts, particularly for aerospace applications.
Semi-permanent mold release agents are also applied to the mold surface. They can be of the wipe-on or spray-on type. These release agents are preferred because they are not completely depleted following a single molding operation. A single semi-permanent mold release agent application can be used to facilitate the release of multiple successive composite molded parts without reapplication of the release agent; e.g. up to 2, 3, 4, 5, 6, 7, 8, 9, or 10, or more releases from a single mold release agent application. U.S. Pat. No. 5,601,641 (incorporated herein by reference) describes one type of semi-permanent mold release agent that has 0.1 to 12% silane, 0 to 16% methyl terminated polydimethylsiloxane emulsified polymer, 0 to 10% non-ionic surfactant, 0 to 8% fluorinated alkyl ester surfactant, 0 to 2.5% ethoxylated amine surfactant, 0 to 12.5% silanol terminated polydimethylsiloxane having a weight average molecular weight of 400 to 310,000, 0.1 to 8% lower alkyl alcohol wherein the alkyl has 1-3 carbon atoms, 0 to 2% fungicide, 0 to 20% metallic salt of an organic acid, 0.5 to 2% weak acid to maintain the pH between 4.5-5.5, 0.1 to 15% hydroxyl terminated polybutadiene, and 40 to 97% water. U.S. Pat. Nos. 5,298,556 and 5,219,925 are in the same patent family as the '641 patent mentioned above, and are also incorporated herein by reference.
U.S. Pat. No. 6,322,850, incorporated herein by reference, describes a similar semi-permanent mold release agent, wherein the silane component is present in a concentration of 0.5 to 12 percent by weight and is selected from the following group of 7 specific silanes and mixtures thereof dimethyldiethoxysilane, methyldiethoxysilane, dimethyldimethoxysilane, diphenyldimethoxysilane, trimethylmethoxysilane, trimethylethoxysilane, and aminopropyltriethoxysilane. The composition also includes a multi-functional polydimethyl siloxane emulsified polymer and a compatible surfactant, together with about 0.1 to 30% by weight of ethanol, remainder water.
The mold release agents described in the above patents are applied and cured at high temperature (e.g. mold temperature of 350° F.). When these release agents are applied at room temperature, they not only provide poor release due to insufficient cure at room temperature, but they can also result in detrimental transfer of the mold release agent to finished composite parts. Also, the above-described release agents do not provide uniform mold surface coverage when applied to the mold surface at ambient temperature (e.g. 25° C.).
U.S. Pat. No. 5,399,310, incorporated herein by reference, describes another mold release agent comprising an oligomeric salt agent and a diluent, where the oligomeric salt agent is a lithium, sodium, or potassium salt, or mixture thereof, of an oligomer of monomeric units or mixture of the said oligomers, wherein the monomeric units are selected from the group consisting of C14 to C24 carboxylic fatty acids. This mold release agent is not beneficially used to release molded composite parts due to interactions between the fatty acid monomeric units and the composite material. Also, this release agent results in significant detrimental transfer to the finished molded parts which interferes with or inhibits effective post bonding. Also, this mold release agent is not suitable for high temperature molding (such as composite molding) because the fatty acid monomeric units will liquefy or decompose at or above about 180° C.
It is highly preferred that mold release agents used for molding composite parts are room temperature curable, especially in aerospace applications, because the molds are prepared at room temperature as explained above. After the mold has been prepared (including provision of the mold release agent and the prepreg resin composite), the mold is heated to elevated temperature, often in a separate oven, typically above 180° C., to cure the resin and form the finished composite part.
Up till now, room temperature curing semi-permanent mold release agents with high thermal stability have been solvent based; meaning they contain significant amounts of volatile organic compounds (VOCs), and have relatively low flash points and high vapor pressures. Examples include the Frekote™ mentioned above and other existing or conventional solvent-based mold release agents. These existing solvent based mold release agents present significant health and safety hazards, both in use and transport.
Accordingly, there is a need in the art for a non-solvent based, thermally stable, semi-permanent mold release agent that provides effective mold release of multiple successive molded composite parts following a single initial application to the mold surface. Preferably, such a mold release agent also will result in no or substantially no detrimental transfer of the mold release agent to the surface of the finished composite part, meaning that the mold release agent will not result in any (or significant) residue on the surface of the composite part that will detrimentally affect adhesive bonding to that surface.